In general, solid, liquid and gas insulating materials are employed for insulating an electrical appliance. Of these insulating materials, the solid insulating materials are most extensively employed. Examples of the solid insulating materials are inorganic insulating materials, organic synthetic high molecular weight insulating materials and compounds thereof. After being used for many years, the insulating materials are subjected to electrical and mechanical deterioration and to thermal oxidation deterioration. As a result, the insulation characteristic of the electrical appliance is decreased, and finally dielectric breakdown is caused in the electrical appliance, which may result in the occurrence of an accident. If such an accident occurs then, a heavy loss is incurred and it requires much expense and time to repair and restore appliance to the former condition. Therefore, for accident prevention and security, there has been a strong demand for a method of detecting degradation in the insulation characteristic of an electrical appliance before dielectric breakdown occurs.
In order to meet this demand, a minute amount of deterioration product dissolved in the liquid in case of a liquid insulating material is detected with a highly sensitive gas analyzer to determine the degree of deterioration; and in case of a gas insulating material, a deterioration product mixed therein is detected with the gas analyzer to determine the degree of deterioration.
However, in case of a solid insulating material, unlike the above-described case, it is impossible to employ highly-sensitive gaseous deterioration product as the measure of deterioration, and therefore the deteriorated solid itself must be used for determining the degree of deterioration. On the other hand, a method in which the non-destructive electrical characteristics of an electrical appliance, such as for instance an insulation resistance, a dielectric characteristic and a partial discharge characteristic are measured to estimate the insulation life of the insulator of an electrical appliance, has been employed by way of trial. However, it should be noted that these characteristics do not satisfactorily correspond to the dielectric breakdown characterstic of the electrical appliance.
Since the insulation life of an insulator of an electrical appliance is determined from its dielectric breakdown voltage, in the present circumstance it is impossible to estimate the insulation life of insulator of an electrical appliance by the conventional method.
The inventors have conducted research on a method of detecting the behavior of the dielectric breakdown voltage of resin insulating materials used in electrical appliances. The breakdown voltage decreases with the lapse of operation time. As a result of the research, they have found that the ratio of the number of hydrogen atoms to the number of carbon atoms in the resin insulating materials varies with the time of use of the appliance and is satisfactorily corrspondent to the dielectric breakdown voltage decrease behavior.